WARSAW, Poland (USA TODAY) - President Obama defended the trade that released Sgt. Bowe Bergdahl last weekend, saying Tuesday that the United States does not leave troops behind on the battlefield.

"Period, full stop," Obama said during a news conference in Warsaw, Poland. "We don't condition that."

“This is what happens at the end of wars.”

President Obama

A reporter asked Obama about the Bergdahl on the first day of a Europe trip designed to re-assure allies of support in the face of Russian aggression in Ukraine. Obama said he will ask Congress for $1 billion to bolster security for Poland and other NATO allies in Europe.

Obama said his team pursued the chance for an agreement brokered by Qatar with Taliban insurgents in Afghanistan who had held Bergdahl in captivity for some five years.

To free Bergdahl, the United States released five Taliban detainees from the prison at Guantanamo Bay, Cuba. Obama said he received assurances from Qatar's government that it would restrict the future movements by these detainees, who have been involved in terrorist operations.

Released on Saturday, Bergdahl had been the sole American prisoner of war in Afghanistan.

The Bergdahl deal followed Obama to Europe as Republicans continued to criticize the prisoner swap, saying it could encourage the Taliban to kidnap more people and seek more releases. They also said the the released Taliban detainees could return to kill.

"These particular individuals are hardened terrorists who have the blood of Americans and countless Afghans on their hands," said Sen. John McCain, R-Ariz.

“These particular individuals are hardened terrorists who have the blood of Americans and countless Afghans on their hands.”

Sen. John McCain

GOP lawmakers also said the Obama administration may have violated the law by providing sufficient notice to Congress on the release of prisoners from Gitmo.

Asked about the releases during a news conference in Warsaw, Obama noted that the United States is ending combat operations in Afghanistan, and accounting for prisoners of war is a normal part of the process.

Said Obama: "This is what happens at the end of wars."

In announcing the new $1 billion defense plan, Obama said security in Europe is the "cornerstone of our own security and it is sacrosanct." He added that "it is a commitment that is particularly important at this point in time."

Leaders in Poland and other members of the North Atlantic Treaty Organization have expressed concern about Russia's intentions after it annexed the Crimea region of Ukraine.

Obama, who along with allies has placed economic sanctions on Vladimir Putin's government, said that "further provocations by Russia will be met by further costs for Russia."

The new aid, targeted mostly for NATO allies would be used for land, sea, and air military exercises and training missions throughout Europe, with an emphasis on the east. More U.S. personnel would be rotated throughout Europe under the plan.

"A persistent U.S. air, land, and sea presence in the region, especially in Central and Eastern Europe, is a necessary and appropriate show of support to allies," said a White House statement on the $1 billion proposal.

These allies contributed to counter-terrorism operations in Afghanistan and elsewhere, and "are now deeply concerned by Russia's occupation and attempted annexation of Crimea and other provocative actions in Ukraine," the statement said.

There would also be help for non-NATO nations on Russia's border, including Ukraine, Georgia and Moldova.

The White House said that "we will continue to take actions that increase the capability, readiness and responsiveness of NATO forces to address any threat."

Obama discussed the new security aid in meetings Tuesday with Polish President Bronislaw Komorowski and Prime Minister Donald Tusk.

The U.S. and Polish presidents will also meet with a group of leaders from Eastern Europe: Bulgaria, Croatia, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Romania and Slovenia.

In greeting Obama at the airport, Komorowski said the United States and Poland are a "brotherhood in arms."

The president is visiting Warsaw to help commemorate the 25th anniversary of free elections in Poland.

On Wednesday, Obama the incoming president of Ukraine, Petro Poroshenko.